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vor 7 Jahren

4-2017

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Fachzeitschrift für Hochfrequenz- und Mikrowellentechnik

RF & Wireless Test &

RF & Wireless Test & Measurement A Product Development Flow for 5G/LTE Envelope Tracking Power Amplifiers, Part 2 ET and DPD Enhance Efficiency and Linearity Figure 12: Simulated AM-AM and AM-PM response plots for a power amplifier driving into compression As a result of the increased PAPR and peak power requirements of LTE-A and carrier aggregation, linearization and efficiency enhancement techniques such as DPD and ET will be even more crucial to the future of cellular standards than they already are for LTE today. DPD enables designers to operate in the efficient yet non linear region of an amplifier while retaining the transmitted-signal linearity required of most digital modulation formats. DPD does not produce dramatic improvements in PA efficiency, but it does improve the quality of a signal that a PA produces when operating at its peak efficiency point. The approach to DPD can range from simple solutions such as National Instruments ni.com/awr Figure 13: Predisortion reverses the nonlinear behavior of the PA 88 hf-praxis 4/2017

RF & Wireless a basic look-up table (LUT) to more complex real-time signal processing approaches. To linearize a PA via an LUT, the measured or simulated output power and phase of the PA must be characterized as a function of input power (Figure 12) (simulations performed in Microwave Office based on shown PA circuit). These measurements produce the AM-AM/AM-PM responses used to create an LUT that relates every input power/phase combination to the power/phase required to produce the desired linear output. By predistorting the input waveform, the PA can essentially be linearized. Figure 13 shows the compression of a nominal (64-QAM) waveform, which produces a constellation where the peak portions of the signal experience less gain than other portions. Thus, by predistorting the waveform such that the higher power symbols are amplified (gain expansion), the nonlinear behavior of the Figure 14: LUT DPD development flow based on (1) full simulation in V SS (2) LabVIEW modeling of the LUT with simulated PA (3) LUT implemented in PXI hardware co-simulation with Microwave Office PA model (4) PXI-based LUT predistortion of waveform driving actual PA PA actually corrects the predistorted waveform. The resulting PA output using a predistorted waveform produces a more reasonable constellation. With NI design and test solutions, the LUT can be developed into hardware through a combination of simulated and measured data, progressively replacing simulation models/results with hardware and measured data (Figure 14). The LUT is initially implemented in V SS and used to predistort the waveform, exciting a simulated PA with nonlinearities represented with a behavioral model using measured or simulated data. Comparison of the original PA input/output power response (showing compression) and the response after predistortion is shown in Figure 15. ET technology enables operators to utilize only as much power as is necessary to provide the amplified output. This technology reduces energy consumption, thus significantly lowering operating costs while providing environmental sustainability. In addition, from the hardware system perspective, this means a smaller form factor, higher reliability due to lower junction temperatures, and much Figure 15: V SS implementation of three simulations of PA (1) without DPD (2) PA with LUT (3) PA with fixed-point LUT and resulting P out vs. P in . 55 hf-praxis 4/2017 89

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